The Collaborative International Dictionary of English v.0.48:
Declension \De*clen"sion\, n. [Apparently corrupted fr. F.
d['e]clinaison, fr. L. declinatio, fr. declinare. See
Decline, and cf. Declination.]
1. The act or the state of declining; declination; descent;
The declension of the land from that place to the
sea. --T. Burnet.
2. A falling off towards a worse state; a downward tendency;
deterioration; decay; as, the declension of virtue, of
science, of a state, etc.
Seduced the pitch and height of all his thoughts
To base declension. --Shak.
3. Act of courteously refusing; act of declining; a
declinature; refusal; as, the declension of a nomination.
(a) Inflection of nouns, adjectives, etc., according to
the grammatical cases.
(b) The form of the inflection of a word declined by
cases; as, the first or the second declension of
nouns, adjectives, etc.
(c) Rehearsing a word as declined.
Note: The nominative was held to be the primary and original
form, and was likened to a perpendicular line; the
variations, or oblique cases, were regarded as fallings
(hence called casus, cases, or fallings) from the
nominative or perpendicular; and an enumerating of the
various forms, being a sort of progressive descent from
the noun's upright form, was called a declension.
Declension of the needle, declination of the needle.